The coating of pharmaceutical tablets, granules, pellets and other discrete particles is commonly conducted in an air suspension film coating apparatus which has been referred to as a Wurster coater and to which U.S. Pat. No. 2,648,609 relates.
In principle the Wurster coater operates by repeatedly cycling the particles through a coating zone into a drying zone using gas streams to direct the particle flow and dry the coated particles.
A variety of modifications of the Wurster apparatus have been proposed in the following subsequent issued U.S. Pat. Nos.: 2,799,241; 3,089,824; 3,117,027; 3,207,824; 3,253,944; 3,241,520; 3,253,944; and 4,117,801.
A typical air suspension film coating apparatus is depicted in FIG. 7 of U.S. Pat. No. 3,241,520.
In operation the uncoated tablets are suspended in a vertical air stream using an air suspension apparatus and a horizontal perforated disc. The tablets are placed on the disc and an air source forces heated air through the perforations causing the tablets to rise in a cylinder that is contained within a large outer cylinder. An atomizing spray nozzle is located within the inner cylinder and provides a constant spray. The tablets rise in the air stream, receive a spray coating then fall gently through the outer cylinder back to the plate where the cycle is repeated until a satisfactory film coat is obtained.
Organic solvents have conventionally been used in this process as a means of solubilizing the film-former, surfactant, plasticizer and other coating ingredients. Lower alkanols (e.g. methanol), methylene chloride, specially denatured alcohols, and chloroform have been generally used for this purpose.
During the course of applying the film coating to the tablets, these organic solvents are evaporated by the heat and forced air so that they do not appear in the final product. This removal of the organic solvents presents certain fundamental problems to the pharmaceutical industry in that (1) the highly volatile organic solvents must be shielded from ignition sources to prevent explosion, (2) the solvent vapors are a threat to the environment as well as to the operators of the equipment, (3) the organic solvents are expensive and, since many are petroleum based products, are becoming scarce and (4) the procedures and equipment required to successfully remove the vapors from the environment are expensive.
To obviate the expense and inconvenience of traditional tablet coatings employing organic solvents, aqueous film coating systems have been resorted to. Use of such aqueous compositions has led to problems in achieving uniform, smooth finishes. It is an object of the present invention to provide an apparatus designed to overcome or minimize such problems. While the principal advantage of the present invention relates to the use of aqueous finishing compositions, the present invention is directed to the apparatus not the finishes and the apparatus can be employed with conventional, organic solvent based finishes as well.